1. Field of the Invention
The present invention relates generally to the field of printing mechanisms and more specifically to an extremely high frequency printing mechanism that finds particularly advantageous use in ink-jet-printing, but which may also be advantageous in application to high speed impact printing.
2. Prior Art
In the last several years, ink-jet printers have been gaining in popularity, particularly for use in conventional office applications and also in the areas of desk-top publishing and computer-aided design. The popularity of ink-jet printers is attributable to a number of significant advantages as compared to more conventional dot matrix printers. Most significantly, is extremely high print quality and reliability, combined with relatively low noise operation, low manufacturing costs and relatively low maintenance costs. All ink-jet printers eject droplets of ink with a diameter that is usually less than about 100 micrometers, onto a medium such as paper or transparencies. There are various ways of producing ink droplets on demand. Two types of actuators have become established in the market, namely print systems which use piezoelectric transducers and those which use thermoelectric transducers. Such "drop-on-demand" printers are typically no larger than conventional dot-matrix printers and are basically no more expensive to manufacture. The fact that they can print at high speeds and can do so in relative silence, makes them highly desirable for use as a printer in an office environment. Systems are available with, typically, from four to sixty nozzles and resolutions range from about seventy-two drops per inch (dpi) to four hundred dpi. Because of their low power requirements, such ink-jet printers have been used as lap top printers. Up until about 1984, the only "drop-on-demand" ink-jet printers available on the market were piezoelectric printers in which a voltage is supplied to a piezoelectric element. This results in a mechanical displacement, which causes a pressure change, such as in an ink tube, thereby forcing one drop of ink to discharge toward the printing medium, such as paper. In one such prior art piezoelectric ink-jet device, a piezoelectric tube surrounds a flexible tube having ink therein, and terminating in a nozzle from which the drop of ink is to be ejected. An electric pulse is applied to the piezoelectric tube, causing the internal cross-section of the tube to contract or expand. The result is local over-pressure or under-pressure which creates pressure waves which accelerate the ink meniscus and causes a droplet to be ejected from the nozzle. Piezo-diaphragm systems operate on exactly the same principles as piezo-tube ink-jet mechanisms, except that the pressure waves are produced by the flexural movements of a plain piezo-disk on a rigid diaphragm. Still another piezoelectric configuration for ink jet printing applications are print mechanisms with piezo-lamellas. A large plurality of piezo-lamellas are arranged in parallel and extend or contract when subjected to voltage pulses. This movement is also transferred to an ink chamber and an ink droplet is ejected as a result.
More recently, piezo-activated ink-jet mechanisms have come under competition from bubble-jet systems. These systems employ an elongated channel in which there is a heating element behind a nozzle. When a short pulse is applied to the heating element, a small vapor bubble is produced in the ink above the element. The bubble drives ink out of the nozzle with great force and then collapses, cutting off the jet of ink. In contrast to piezo systems in which the defined oscillation of the actuator causes the ink to move, the bubble jet relies solely on capillary action to draw the ink into the nozzle.
One of the critical performance characteristics of all such prior art "drop-on-demand" ink jet systems is the maximum ejection frequency attainable and the ink compatibly with the media (paper) and the channel and nozzle. Solid ink avoids all the problem associated with ink drying up in nozzle. Hot-wax-based solid ink solidifies on contact with the paper and is water resistant, provide maximum contrast and most important does not require special paper and can be used with transparencies. This frequency for piezo tube systems is about 5 kHz. For diaphragm operated piezo systems, it is about 3 kHz. For lamella piezo systems, it is believed to be as high as 10 kHz. However, there is no current system on the market which uses the piezo-lamella concept that is capable of a 10 kHz ejection frequency or ejection of hot-melt-ink. The bubble systems have a maximum ejection frequency of about 4 kHz. Thus, of all of the prior art piezo-electric and bubble ink-jet systems on the market, the maximum ejection frequency is 5 kHz. The ejection frequency is a critical parameter of ink-jet systems because for the same number of nozzles, it determines how quickly a character can be formed, or alternatively, in color ink jet printing applications, it determines how quickly different color combinations can be created, and thus whether or not a color ink-jet printing system can be made that is feasible in terms of print speed. Another critical performance characteristic is resistance to clogging ink channels which can readily occur with long ink channels.